external_plugin_deps.bzl: pass buildifier and add module comment

Fix buildifier warnings.

Change-Id: Ic7332745576c480eaa8dad308a39d1265226822c
diff --git a/external_plugin_deps.bzl b/external_plugin_deps.bzl
index 0bd5c63..3f992ea 100644
--- a/external_plugin_deps.bzl
+++ b/external_plugin_deps.bzl
@@ -1,17 +1,21 @@
+"""
+External dependencies of the supermanifest plugin
+"""
+
 load("//tools/bzl:maven_jar.bzl", "maven_jar")
 
 def external_plugin_deps():
-  maven_jar(
-    name = 'jaxb-api',
-    artifact = 'javax.xml.bind:jaxb-api:2.3.1',
-  )
+    maven_jar(
+        name = "jaxb-api",
+        artifact = "javax.xml.bind:jaxb-api:2.3.1",
+    )
 
-  maven_jar(
-      name = 'istack-commons-runtime',
-      artifact = 'com.sun.istack:istack-commons-runtime:3.0.11'
-  )
+    maven_jar(
+        name = "istack-commons-runtime",
+        artifact = "com.sun.istack:istack-commons-runtime:3.0.11",
+    )
 
-  maven_jar(
-      name = 'jaxb-runtime',
-      artifact = 'org.glassfish.jaxb:jaxb-runtime:2.3.3'
-  )
+    maven_jar(
+        name = "jaxb-runtime",
+        artifact = "org.glassfish.jaxb:jaxb-runtime:2.3.3",
+    )